This invention relates to electric heating of motor vehicle windows to remove or prevent the formation of ice and fog, and more particularly to a method of regulating the electric power supplied to the heaters.
Many motor vehicles are equipped with electrically powered heaters for preventing or quickly removing fog and ice from the rear window, and in some cases from the front window (windshield). Rear window heaters are generally manufactured by forming a long serpentine conductor pattern on the glass, whereas front window heaters are generally manufactured by depositing a very thin film of conductive material on the glass. In either case, electric current is supplied to the heater to initiate heating in response to activation of a driver-operated switch, and the current is maintained for a predetermined interval, after which the heater is turned off to conserve power. Under most conditions, the heating interval is adequate to remove ice or fog, but under more severe conditions, the driver may need to reactivate the heater to obtain sufficient heating.
Although most rear window heaters have relatively modest power requirements (300W to 400W), front window heaters typically have much higher power consumption (1000W) and pose a significant burden on an ordinary vehicle electrical system, particularly under engine idle conditions when the alternator output is relatively limited. In fact, the combined electrical load of the front and rear window heaters may exceed the alternator capacity and seriously discharge the storage battery.
The above-mentioned drawbacks can be alleviated to some degree by installing a moisture sensor on the front and/or rear windows, and automatically activating the respective heaters only when fog or ice is actually present. In this vein, the U.S. Pat. No. 5,653,904 to Adiparvar et al. discloses a system for automatically activating a rear window heater when moisture or dew is detected on the rear window, and for automatically activating the defrost mode of the vehicle heating and air conditioning system when moisture or dew is detected on the front window. However, the problem of excessive power consumption can still occur, and there is no provision for activating the heaters to take preventative action against fogging. Accordingly, what is needed is a control for automatically activating the window heaters at a controlled activation level that eliminates and/or prevents the formation of ice and fog without over-taxing the vehicle electrical system.
The present invention is directed to an improved method of electric window heater activation wherein front and rear window heaters are automatically and independently activated at a variable level based on the respective potential of fogging, within the ability of the vehicle electrical system to supply the requested current without discharging the storage battery. According to the invention, a defog controller develops front and rear fog factors indicative of the relative potential of fogging, and activates the respective electric heaters as required to drive the respective fog factor to zero. The fog factors are based on an estimate of the cabin air dewpoint temperature, the temperature of the respective window surfaces, and a temperature interval over which the fog factor signals only partial activation of the respective heater. The temperature interval is biased in a direction to provide preventative activation of the heaters at a relatively low level when the electrical power requirement is limited, and the activation level is limited as required to prevent battery discharging.